With speeding-up and large-capacity, in recent optical communication systems, the bandwidth equal to or broader than 40 GHz is demanded. Accordingly, also in an optical modulator used in an optical transmitter or the like, wider bandwidth of equal to or broader than 40 GHz is demanded. A recent optical modulator adopts a Mach-Zehnder optical modulation device in which crystal having an electro-optic effect such as lithium niobate (LiNbO3) serves as a substrate, and an optical waveguide is formed in the substrate, and an electrode is formed on the substrate. Therefore, such an optical modulation device is required to be operable in a band equal to or broader than 40 GHz.
FIGS. 11 and 12 illustrate a plan view of an optical modulation device using the lithium niobate substrate (LN substrate) and an A-A cross section thereof, respectively. In an inner portion under a surface of the LN substrate 1, an optical waveguide 2 is formed by a method of thermally diffusing from a patterned metal film such as titanium or the like, a method of proton exchanging in benzoic acid after patterning the metal film, or the like. The optical waveguide 2 has a structure including parallel two branching portions 2c and 2d arranged between an incidence end portion 2a and an emission end portion 2b An electrode 3 for modulating a light traveling over the foregoing optical waveguide 2 is formed on the LN substrate 1 via a buffer layer 4 of low refractive index, such as a SiO2 film. The electrode 3 is formed to act on the branching portions 2c and 2d , and includes a signal electrode 3a formed on the one branching portion 2c and earth electrodes 3b on both sides of the signal electrode 3a. 
The signal electrode 3a is a traveling-wave electrode for applying an electric field to the optical waveguide 2. Namely, a high frequency electric signal in accordance with transmission data is input from one end of the signal electrode 3a and the other end of the signal electrode 3a on the opposite side is terminated at 50 ohm or the like. The widths of and the spaces between the signal electrode 3a and the earth electrodes 3b are designed so that a speed of the electric signal is matched with a speed of the light.
In the case where the foregoing optical modulation device is used in the broadband equal to or broader than 40 GHz, there is a problem in that the high frequency is leaked out into the LN substrate to become another mode, so that the dip (rapid degradation of characteristics) occurs in frequency characteristics. With regard to this, FIG. 13 illustrates frequency transmission characteristics (S21) of the optical modulation device. As illustrated in FIG. 13, the band of the optical modulation device depends on the dip occurrence in the high frequency transmission characteristics, and therefore, it is hard to operate the optical modulation device in the broadband equal to or broader than 40 GHz. As illustrated in FIG. 14, it is considered that such dip occurs as a result that the high frequency electric field leaked out into the LN substrate 1 is coupled to another mode such as a TE mode, a TM mode or the like, and it is known that the frequency occurring the dip differs depending on the thickness or width of the LN substrate 1.
Techniques of suppressing the above-mentioned dip in the optical modulation device using the crystal substrate having the electro-optic effect are disclosed in the following reference literatures 1 to 6.    Reference literature 1: Japanese Laid-open Patent Publication No. H05 (1993)-257104    Reference literature 2: Japanese Laid-open Patent Publication No. 2002-169133    Reference literature 3: Japanese Laid-open Patent Publication No. H05 (1993)-093892    Reference literature 4: Japanese Laid-open Patent Publication No. 2002-357797    Reference literature 5: Japanese Laid-open Patent Publication No. 2005-181537    Reference literature 6: Japanese Laid-open Patent Publication No. H05 (1993)-276701
The reference literature 1 discloses that reducing a thickness of a substrate, to thereby shift the frequency occurring the dip to the frequency higher than the electric signal to be applied on the signal electrode. However, in the case of applying this technique, the thickness of the substrate which is normally about 0.5 mm to 1 mm, needs to be decreased to 0.2 mm or less in order to ensure the band equal to or broader than 40 GHz. Accordingly, the sufficient mechanical strength of the substrate is lacked. Namely, there is a possibility that the substrate is broken during the manufacturing process, or even after mounting in a module, the substrate is broken due to a difference between the thermal expansion thereof and that of a case, a terminal substrate or the like, and therefore, the process yield may be lowered.
The reference literature 2 discloses that making a groove on a rear surface of a substrate to form a thin portion, to thereby decrease a voltage of an electric signal applied to an electrode. In the case of applying this technique, the thin portion of 0.1 mm or less is partially formed on the substrate of normally 1 mm thickness, and therefore, the sufficient mechanical strength of the substrate is still lacked. Further, an adhesive used for attaching the substrate enters into the groove portion, and therefore, there is a possibility to occur the substrate breaking due to a difference between a thermal expansion coefficient of the substrate and that of the adhesive.
The reference literature 3 discloses that attaching a thinned substrate onto a glass substrate of low dielectric constant to enhance the mechanical strength. However, a thermal expansion coefficient of the low dielectric constant substrate such as glass substrate is inconformity with that of the electro-optic crystal substrate (e.g., the thermal expansion coefficient of LN substrate; 15 ppm/deg C. relative to the thermal expansion coefficient of SiO2; 0.5 ppm/deg C.), and accordingly, there is a possibility that the substrate is broken due to the thermal expansion difference. Further, the substrate breaking during processing of making the substrate to be thinned before attaching on the low dielectric constant substrate is not negligible.
The reference literature 4 discloses that attaching a thin substrate onto a case formed with a concaved space to use the concaved space as a low dielectric layer. Also in the case of applying this technique, in order to sufficiently suppress the dip, the substrate needs to be thinned, and accordingly, the substrate breaking during the manufacturing process may not be solved.
On the other hand, the reference literatures 5 and 6 disclose techniques different from the above-mentioned techniques such as making the substrate to be thinned. The reference literature 5 discloses that forming a conductive floating electrode on a side portion of a substrate to suppress a resonance of the substrate with specific frequency, to thereby avoid the dip. Since the substrate is not made to be thinned in this technique, this technique is more preferable than the above-mentioned techniques in view of the mechanical strength. However, the dip is not suppressed but is dispersed over the frequencies in the technique of the reference literature 5, and therefore, the band degradation easily occurs compared with the above-mentioned techniques of making the substrate to be thinned. Further, the reference literature 6 discloses that forming a substrate shape non-uniformly in the thickness and width to disperse the dip. However, also in this technique, the dip is not suppressed but is dispersed, and accordingly, the band degradation is not negligible.
As described in the above, there has not yet been proposed a technology capable of achieving both of the dip suppression and the mechanical strength of the crystal substrate having the electro-optic effect, for corresponding to the broadband equal to or broader than 40 GHz. Therefore, it is considered that an optical modulation device having a structure capable of suppressing the dip without lowering the mechanical strength and a manufacturing method for the optical modulation device are required hereafter.